The invention in question concerns an actuating device, consisting of a hydraulic slave-cylinder integrated with a hydraulic pump, which with the assistance of a linear electric motor drives the pump piston in a reciprocating motion within a pump cylinder thereby building up hydraulic pressure to the level desired for driving a piston in the slave cylinder, thus enabling a piston rod connected to the piston in the slave cylinder to execute mechanical work.
Linear hydraulic actuators are used today in, for example, the aircraft industry, where they are used for manoeuvring flight control surfaces such as ailerons. The type of actuator referred to here is also used in railway construction, for example for tilting railway wagon bodies through curves. There are of course numerous other applications found in a range of technological areas. Applications within the aircraft industry have been used in this document to describe the current technology along with its possible limitations. For example, aircraft require a number of operational arms, which with the huge forces necessary manoeuvre their control surfaces for executing, for example, changes in flight paths and landings. Normally these structures used for manoeuvring aircraft are constructed using hydraulic cylinders, which by means of transferring a desired pressure through fluid in hydraulic lines bring about the manoeuvring of the control surfaces to predetermined positions. Hydraulic cylinders are preferable in these situations where great actuating forces may be required. However the use of numerous hydraulic slave cylinders distributed throughout an aircraft requires the installation of a large, central hydraulic system. Lines for carrying hydraulic fluid have to be run to each individual hydraulic manoeuvring device. All in all, this means that rather complicated hydraulic systems must be fitted onboard aircraft. Lengthy and numerous hydraulic lines have to be run along the aircraft fuselage and wings. Redundant systems have to be fitted to combat any drop in hydraulic pressure somewhere in the system, so that the manoeuvrability of the plane is not jeopardised. Having hydraulic lines running throughout an aircraft increases the risk of fire, as hydraulic fluid may leak into sensitive areas. The hydraulic system accounts for an increase in the total weight of the aircraft, which could be reduced if alternative technology was available. Moreover, hydraulic systems are expensive and increase the probability of failure. Serviceability is also reduced since the hydraulic system is distributed throughout the craft, where certain parts can be difficult to replace. Perhaps the greatest weakness of hydraulic systems in this context is the difficulty encountered actually installing hydraulic manoeuvring systems of the type mentioned here within aircraft or for that matter any sort of technical machine. There is a desire within the industry today to replace the central hydraulic system with an electrical system. This has not been possible yet, as actuators suitable for these purposes are not yet available.
In accordance with the invention, an electrohydraulic actuating device is presented, consisting of a hydraulic slave cylinder containing a piston that pushes a piston rod and which receives its pressure for building up this piston-derived power via its integration with a hydraulic pump, which is in turn is driven by a linear electric motor.
The hydraulic pump consists of a pump cylinder in which a piston slides at a specified frequency in a reciprocating motion within the pump cylinder. The piston""s motion is controlled through its functioning as the rotor for the linear electronic motor, with the motor""s stator coil being wound around the pump cylinder itself. Moreover, the piston functions as a pump piston for pumping hydraulic fluid to the slave cylinder. An additional feature is that the piston also functions as a valve for opening and closing compression and suction ducts for carrying hydraulic fluid to and from the slave cylinder during the piston""s reciprocating motion. By pumping hydraulic fluid at a high frequency, high pressure can be built up and great force exerted on the slave piston in the cylinder, even when the hydraulic pump is small.
At each end of the pump cylinder there is a coil that together act to rotate the piston a half rotation around the pump cylinder""s central axis. This ensures that the functioning of the hydraulic-fluid filled suction and compression ducts linking the pump cylinder and the slave cylinder is not disrupted with every stroke of the pump piston, due to the change in stroke direction with every new stroke. For this reason, the compression ducts have at least two outlets, and the suction ducts at least two inlets. Due to the way the piston functions as a valve as it rotates a half revolution round its central axis, a first opening into the compression duct, which functions as an outlet during the pistons motion in a first direction, will be closed by the piston""s movement in a second direction, at the same time as a second opening into the compression duct opens, and functions as a second outlet. Correspondingly, due to the way the piston functions as a valve as it rotates a half revolution round its central axis, there is a first opening into the suction duct, which functions as an inlet during the piston""s motion in a first direction, and will be closed by the piston""s movement in a second direction, at the same time as a second opening into the suction duct opens, and functions as a second inlet. In this way the strokes of the pump piston work together to build up the desired hydraulic pressure on one side of the slave piston in the slave cylinder regardless of the direction of the pump stroke. The rotating of the pump piston by means of the coils at the ends of the pump cylinder works essentially the same way as a stepping motor. In principle, a number of inlet and outlet ducts can be distributed around the pump cylinder""s periphery in a regular arrangement, for example at 120xc2x0 intervals, with the piston cavities aligned according to the distribution of ducts, which would in this case mean that the piston would only have to advance a third of a rotation with every stroke.
Because of the high working frequency and the large ratio between the cross-sectional area of the piston in the slave cylinder and the cross-sectional area of the piston in the pump, a high exchange of power can be achieved between the pump and the slave piston. This enables great actuating forces to be achieved from a very spatially economic and weight-effective manoeuvring device in the form of the actuating device being described here. As a result of this, it would be possible to achieve a distributed system of local hydraulic actuators, in a set-up such as a craft, for manoeuvring mechanical elements in a craft, where the actuators are driven and controlled electrically. Consequently, the need for a network of hydraulic lines and central hydraulic systems characteristic of current technology previously mentioned could be done away with.
In those applications where a mechanical element, for example an aileron, manoeuvred by an actuator is affected by other external forces so that the element is forced back to a neutral position, these external forces can be utilised for regeneratively re-supplying the power back to the actuator""s electrical supply. This would reduce the cooling requirements of the hydraulic fluid in the actuator, thus contributing to the energy efficiency of the set-up.
The linear motor""s principal coil, that is the stator coil, may consist of a simple coil, though it is advantageous to use multi-phase winding. This can make it easier to steer the forces during the piston stroke in each respective direction, and therefore facilitate optimisation of the actuator""s efficiency.
The winding current is controlled by a static converter, preferably integrated with the hydraulic pump, for example along the lines of the well-known integral motor concept.
In those cases where it is desired that the slave piston rod in the actuator moves in the opposite direction to that described above, this can be easily achieved by having the pump piston position around the central axle work in reverse phase, i.e. that if the pump piston with its movement in one direction has a rotational position v while working to drive the rod outward from the slave cylinder, then the piston""s action with movement in the same direction but with the piston rotation position advanced to xcexd+ the rotation interval (depending on how the inlet and outlet ducts are distributed around the pump cylinder), will bring about pump work where the slave piston rod is forced inwards into the slave cylinder instead of outwards.